1. Field of the Invention
This invention relates to metallic thin film magnetic recording media and methods for forming metallic thin film magnetic recording media.
2. Description of the Prior Art
It is well known in the art of magnetic recording that magnetic disks using magnetic metallic thin film media exhibit a higher recording density than disks utilizing conventional iron oxide or other metallic particles suspended in a binder. This is because metallic magnetic thin films can be made to have a much higher saturation magnetic moment (and hence a higher magnetic remanent) and can be made much thinner than the binder type media. Typical films of magnetic media include cobalt alloys such as Co-Ni, Co-Ni-Cr, Co-Pt, Co-Ni-Pt, Co-Sm, and Co-Re formed by vacuum deposition.
In order for the magnetic media to be able to support a high linear recording density, it is necessary to minimize the film thickness (e.g., below 1000.ANG.), and the magnetic coercivity H.sub.c must be high enough to sustain a high density of flux reversals per linear inch. For example, to sustain a packing density of ten thousand to several tens of thousands of flux reversals per inch, the coercivity should be between 600 and 2000 Oersteds. Furthermore, if the media is to be used in high density recording, the media should be capable of providing a strong output signal during the read-back process under high packing density conditions.
In order for the media to provide a strong output signal during read-back, the media should have an intrinsically high saturation magnetization M.sub.s, e.g., greater than 400 electromagnetic units per cubic centimeter (emu/cc) and preferably greater than 800 emu/cc. The media should also have a high hysteresis loop squareness S (S=M.sub.r /M.sub.s), e.g., at least 80% to provide a high magnetic remanent M.sub.r. The strength of the magnetic field received by the disk reading head (or read/write head in the case of a single inductive head used for both writing and reading) is proportional to the film thickness T times magnetic remanent M.sub.r. M.sub.r T should generally be greater than approximately 2.times.10.sup.-3 emu/cm.sup.2 to provide a sufficiently strong signal for a practical disk drive.
The above-described magnetic parameters are discussed in greater detail in an article entitled "Thin Film for Magnetic Recording Technology: A Review" by J. K. Howard, published in the Journal of Vacuum Science and Technology in January, 1985, incorporated herein by reference.
In magnetic recording, data is stored or recorded in the magnetic media by forming a sequence of small cells of oppositely polarized magnetized regions. The size of the regions is determined by the width of the inductive writing head and is typically on the order of one micron long and over ten microns wide. A "bit" of stored information is represented by a transition between two oppositely polarized magnetized regions. The transition between two oppositely magnetized regions has a certain characteristic physical length due to the demagnetization effect that occurs in the transition region as the result of the two opposite magnetic dipoles being present. This characteristic length of the transition region is generally called the transition length L and the minimum transition length L.sub.min can be related to the media parameters in the case of saturation recording based on an arctangent transition model as described in "Analysis of Saturation Magnetic Recording Based on Arctangent Magnetization Transition" by R. I. Potter in J. Appl. Phys., Vol. 41, pages 1647-1651, 1970, or "A Theory of Digital Magnetic Recording on Metallic Films" by P. I. Bonyhard, et al., published in IEEE Trans Magn., Vol. MAG-2, pages 1-5, 1966, incorporated herein by reference. This model can be summarized as follows: EQU L.sub.min .apprxeq.2.pi.M.sub.r T/Hc (1)
where M.sub.r is the remanent magnetization, T is the magnetic film thickness, and H.sub.c is the coercivity of the magnetic film. This equation was derived based on a macromagnetic theory and gives a relationship between the characteristic transition length L and the media parameters. Media having a small characteristic transition length L is capable of an increased linear packing density due to reduction in bit crowding by adjacent bits.
It should be noted that even though equation 1 qualitatively predicts the dependence of transition length L (and, hence, the packing density) on media thickness, remanent and coercivity of media, it does not do so quantitatively for thin metallic film media. This is due to the fact that equation 1 was derived based on macromagnetic considerations without taking into consideration the state of complex micromagnetic domains which exist in the transition region.
Many papers examining micromagnetic structures report that the transition region in the metallic thin film media has a "sawtooth-like" jagged boundary micromagnetic structure rather than the "domain-wall-like" smooth boundary used in the theoretical computation of the equation 1. (See, for example, "Transition Region in Recorded Magnetization Pattern," by Curland, et al., published in J. Appl. Phys, Vol. 41, pages 1099-1101, 1970; "Electron Microscopy on High Coercive Force Co-Cr Composite Film," by Daval, et al., published in IEEE Trans. Magn., Vol. MAG-6, pages 708-773, 1970, and "A Study of Digitally Recorded Transition in Thin Magnetic Film," by Dressler, et al., published in IEEE Trans. Magn., Vol. MAG-10, pages 674-677, 1974, incorporated herein by reference.) The origin of the jagged "sawtooth-like" transition is associated with the formation of magnetic clusters that are caused by strong interparticle interaction of the magnetic crystalline particles, as reported by Chen et al. in "High Coercivity and High Hysteresis Loop Squareness of Sputtered Co-Re Thin Film," J. Appl. Phys., Vol. 50, pages 4285-4290, 1979, incorporated herein by reference. (As used herein, the term "magnetic cluster" is a portion of the media consisting of many grains having a nearly constant magnetization direction.) These magnetic clusters cause the formation of complex vortex-like domain structures in the transition region and, hence, the "sawtooth-like" boundary appearance, as reported by Chen in "The Micromagnetic Properties of High-Coercivity Metallic Thin Films and Their Effects on the Limit of Packing Density in Digital Recording," IEEE Trans. Magn., Vol. MAG-17, pages 1181-1191, 1981, incorporated herein by reference.
The formation of the vortex micromagnetic domain structure in the transition region and "sawtooth-like" ragged structure in the boundary of the transition region not only increases the transition length (and, hence, reduces the capability of supporting a high packing density), but also causes excessive media noise and bit shift. (Bit shift occurs when a transition region between magnetized cells is displaced from a desired position. If the transition region is displaced by too great a distance, data on the disk may be misread.) To reduce the media noise, bit shift, and transition region length caused by the jagged transition region, it is necessary to develop a disk manufacturing process to produce media which minimizes the size of the jagged transition region. To achieve this goal, it is necessary to control the crystal growth and morphology of the microstructure of the magnetic crystalline material during the disk manufacturing process.
Another factor which may degrade media performance by causing excessive noise and bit shift is the formation of small abnormal regions having magnetic properties different from those of other portions of the disk surface. As previously mentioned, data stored in magnetic disks is typically recorded in a sequence of small cells (or magnetized regions) which are typically on the order of one micron long and ten microns wide. A typical high performance 51/4" diameter rigid disk using thin metallic film media stores more than 10.sup.8 bits on each surface of the disk. If the magnetic film includes an abnormal region having a size on the order of the above-mentioned cell size (i.e., where the magnetic properties of the abnormal region are different from the magnetic properties of the rest of the film), then the recording characteristics in the abnormal region will be different from the recording characteristics of the rest of the disk surface. Such abnormal regions lead to poor disk performance.
As an example, if the coercivity H.sub.c and hysteresis loop squareness S in an abnormal region of the disk are significantly different from the coercivity and squareness in the surrounding area of the disk, the state of magnetization and the position of the transition region of each cell in that abnormal region may be different from those in the surrounding normal area even under constant writing waveform conditions. This can be particularly true at or near the junction between the normal and abnormal areas where the transition between cells may be shifted away from a desired position due to the difference in the coercivity H.sub.c between the normal and abnormal regions. Consequently, the bit stored in the abnormal area will exhibit excessive bit shift and result in a reduced signal detection window margin. (The signal detection window is a time window during which a transition must be detected in order for data to be read properly. The signal detection window margin is the amount of time between the detection of a transition region and the edge of the signal detection window.) Furthermore, the signal-to-noise (S/N) ratio, resolution and overwrite characteristics in the abnormal region will be different from those of the surrounding area.
A typical example of how an abnormal region can cause noise in a read-back operation occurs when the coercivity H.sub.c in the abnormal region of the disk is significantly higher than the average coercivity across the disk surface. In this case, the magnetization in the cell may not be saturated by the writing head and, hence, the output signal amplitude from the reading head during the read-back process in the abnormal area would be lower than the output signal provided by the rest of the disk. In addition, the overwrite of the high H.sub.c abnormal region may be poorer and consequently the S/N ratio from the abnormal region would be lower than the S/N ratio for the rest of the disk. Conversely, if the coercivity in the abnormal region is lower than the average disk coercivity, then the resolution exhibited by the media at the abnormal region would suffer.
In typical commercial disk drives, the read/write head is optimized for a specific coercivity and such disk drives cannot tolerate deviation in coercivity of more than plus or minus 50 Oe. A deviation of coercivity beyond the prescribed limits usually manifests itself as a phase margin error (i.e., a soft error) which is caused by a shift in the disk output signal pulse (indicative of a transition region) outside of the signal detection window.
If the hysteresis loop squareness S changes locally, the magnetic remanent M.sub.r (which is proportional to the amplitude of the output signal) changes as well, leading to signal modulation. If the remanent at the abnormal region is lower than the average disk remanent due to poor hysteresis squareness, the output signal amplitude provided by the abnormal region may be too low. It is therefore necessary to ensure that there are no regions on the disk larger than a few square microns having magnetic characteristics which greatly differ from the magnetic characteristics which of the disk.
The magnetic properties of thin film media are closely related to the film microstructure (e.g., to parameters such as grain size, crystal phase, amount of grain separation and grain orientation). It is generally accepted that control of the nucleation process is often the key factor in determining the ultimate structure of a vacuum deposited magnetic film. In turn, the composition, crystalline structure, and morphology of the substrate surface upon which the film is formed are key factors for controlling the nucleation process. Thus, magnetic films formed under identical process conditions can have entirely different microstructures, and thus different magnetic properties, depending upon the surface condition of the substrates on which the films are formed.
The surface characteristics of substrates used to form magnetic disks can be affected by a number of factors. For example, a substrate surface may be contaminated by absorbing or reacting with disk cleaning agents or water during substrate cleaning or ambient gaseous elements during the substrate drying process. Such contamination can affect the crystal structure of the magnttic film deposited on the substrate and thus affect the film magnetic characteristics.
Furthermore, substrate surfaces are typically polished and textured prior to magnetic film deposition to provide aerodynamic characteristics which enable the read/write head to fly. Such polishing and texturing can induce strain and change composition on a microscopic scale and hence affect magnetic film crystal growth. If the contaminants and surface strain are not distributed uniformly over the entire disk surface or the substrate composition varies over the disk surface, the resulting film will exhibit nonuniform magnetic properties and will include regions having abnormal magnetic characteristics.
Another factor which can cause the magnetic film to exhibit nonuniform magnetic characteristics is nonuniformity of the crystal structure or morphology of the underlying substrate in the case of crystalline substrates.
There are two methods currently used to alleviate nonuniformities in disk substrates. One method is to sputter-etch or plasma-etch the substrate just prior to magnetic filme deposition. During such processes, a portion of the substrate surface is removed. Unfortunately, the above described etching techniques are difficult to implement because they require application of a negative voltage to the substrate, and it is difficult to apply the negative voltage uniformly across the substrate surface.
The second method for alleviating the effects of nonuniformities in the substrate surface is to sputter a thick chromium layer onto the substrate just prior to forming the magnetic film. Such processes are typically used in conjunction with magnetic films comprising Co-Ni or Co-Ni-Cr alloys. In such processes, the primary purpose of the chromium layer is not to mask nonuniformities in the substrate surface, but rather to control and increase the coercivity of the Co-Ni or Co-Ni-Cr alloy. (See, for example, "Thin Film Memory Disc Development" by Opfer, et al., published in the Hewlett Packard Journal, in November, 1985, incorporated herein by reference.) However, as a secondary effect, the chromium does partially mask nonuniformities in the substrate surface. The chromium layer comprises crystalline grains which are influenced by the substrate surface morphology. Thus, the chromium crystal structure can be significantly affected by substrate surface condtions and can, in turn, transmit the substrate surface conditions to the magnetic film. Thus, even when using a chromium layer between the substrate and magnetic media, nonuniformities in the substrate surface can still create nonuniformities in the magnetic properties of the media.
In summary, magnetic recording media using metallic magnetic thin film can provide higher linear packing densities. However, due to nonuniformities in the substrate surface, small abnormal areas exhibiting abnormal magnetic properties may be created. As a result, excessive bit shift and media noise can occur and degrade the media performance for high density recording. In addition, although the theoretical transition length of the thin film media may be small in comparison to the binder type media, "sawtooth-like" jagged transition regions cause excessive bit shift and media noise during high density recording. These jagged transition regions also limit the recording density achievable with thin metallic magnetic films.
To overcome these problems, it is desirable to create a metallic magnetic film having a uniform crystalline structure across the whole surface of the disk and the structure and morphology of the crystalline structure should be tailored to reduce the jagged transitions.